The process of TW-electroerosion generally makes use of a continuous electrode wire which is typically circular in cross section and composed of, say, brass or copper, or brass- or copper-coated steel, the wire having a thickness or diameter generally ranging between 0.05 and 0.5 mm. The electrode wire is axially transported continuously from a wire supply to a wire takeup through a cutting region in which a workpiece is disposed. The cutting region is commonly defined between a pair of guide means which support and hold the wire while traveling through the workpiece. Wire traction and braking means allow the continuous wire to be tightly stretched and kept taut under a given tension and to be axially driven between the cutting guide members while traversing the workpiece, thus presenting a continuously renewed electrode surface juxtaposed in an electroerosive cutting relationship with the workpiece across a narrow gap or cutting zone. The cutting zone is flushed with a cutting liquid medium, e.g. water, and is electrically energized with a high current density electrical machining current which is passed between the electrode wire and the workpiece to erode the latter or erosively remove material therefrom.
The cutting process may be performed in any of various electroerosive machining modes. In electrical discharge machining (EDM), the cutting liquid is a dielectric liquid, e.g. deionized water, and the machining electric current is supplied in the form of a succession of electrical pulses. In electrochemical machining (ECM), the cutting medium is a liquid electrolyte, e.g. an aqueous electrolytic solution, and the machining current is a high-amperage continuous or pulsed current. In electrochemical-discharge machining (ECDM), the liquid medium has both electrolytic and dielectric natures and the machining current is preferably applied in the form of pulses which facilitate the production of electric discharges through the conductive liquid medium.
The workpiece may be disposed in a bath of the cutting liquid medium to immerse the cutting region therein. More typically, however, the cutting zone is disposed in the air or ambient environment. Advantageously, one or two nozzles of a conventional design are disposed at one or both sides of the workpiece to deliver the cutting liquid medium to the cutting region disposed in the air or immersed in the liquid medium. The cutting liquid medium is conveniently water as mentioned, which is deionized or ionized to a varying extent to serve as a desired electroerosive cutting medium.
To advance electroerosive material removal in the workpiece, the latter is typically displaced relative to the traveling wire and transverse thereto. This allows the travelling wire to advance translationally in the workpiece and consequently permits a narrow cutting slot to be progressively formed behind the advancing wire, the slot having a width substantially greater than the thickness of the wire. The continuous relative displacement along a precision-programmed path results in the formation of a desired contour corresponding thereto and defined by this cutting slot in the workpiece.
Higher cutting speed in the process described is ever an increasing demand in the industry and is desirable to achieve with due precision. The cutting speed, typically expressed in mm.sup.2 /min, is defined by the product of the workpiece thickness and the length of cut achieved per unit time along a given course and hence is, for a given workpiece thickness, dependent upon the rate of translational advance of the wire electrode that can be increased. The rate of advance is in turn limited by the rate of actual material removal dependent on the one hand upon the preset cutting parameters that govern, inter alia, the cutting accuracy and on the other hand upon the conditions in the cutting zone which may instantaneously vary. If the rate of advance happens to exceed the rate of actual material removal, the fine wire comes to break. The goal of higher cutting speed is, therefore, dependent on the extent to which optimum conditions in the cutting zone may be established and may be maintained stably against instantaneous changes.
In the interest of increasing the cutting accuracy in the TW-electroerosion process, it has been commonly recognized in the art that each of the wire guides disposed at opposide side of the workpiece have advantageously a so-called "die guide" structure with a small opening adapted to guidingly accept the traveling wire therein. A "die" opening is circular to conform with the wire which is circular in cross section as is common. Since the traveling wire stretched under tension remains confined within the opening, it does not come off fatally. When this structure is applied to the two guides, the centers of these openings establish the straight line for the axis of the wire when traveling through the cutting zone to coincide with. As can be seen, it is desirable that the clearance between the wire and the wall of the guide opening be minimized so that the wire axis may have suffer a minimal deviation from the straight line established by the centers of these openings. On the other hand, the guide opening must be large enough to allow the wire to be readily through. Thus, for example, for a wire of 0.2 mm diameter, a guide opening of a diameter as large as 0.21 mm has often been found to be inadequate.
The need for larger clearance also arises especially with respect to the "die" guide at the downstream side of the cutting zone. The wire passing out of the cutting zone has its surface substantially roughened and small pits and projections formed thereon as are typical in rough EDM operations. If such projections are large enough, the wire will be caught in the opening so that its length in the cutting zone becomes slack, causing a short-circuit with the workpiece. Then the wire will break.
As can readily be seen, the larger the clearance, greater the guiding inaccuracy and hence the inferior the machining accuracy. In an electroerosive cutting operation, there developes in the cutting zone a machining pressure (e.g. due to machining discharges) in the direction which is opposite to the cutting direction, thus tending to deflect the wire backwards. As a result, the wire in each of the guide openings will be forced to assume a deviated position with respect to the cutting direction, in spite of high tension applied to the wire to maintain it as straight as possible across the cutting zone. It can therefore be seen that there results a wire guiding inaccuracy and hence a machining inaccuracy which proportionally increases with an increase in the clearance or in the diameter of each guide opening.